Field of the Invention
The present invention relates to determining the state of charge of a battery and more particularly to determining the state of charge at any time regardless of whether or not the battery is in use.
State of the Art
The installed base of portable equipment requiring reliable power from a primary (disposable) or secondary (rechargeable) battery is expected to grow dramatically during the next several years driven by the proliferation of such devices as the laptop computer and cellular telephone. The demand for both primary and secondary batteries for portable laptop computers is expected to triple within the next five years. During the same period, the demand for batteries for use in cellular telephones is expected to grow tenfold or more. Both for the convenience of users of battery operated equipment and to obtain the greatest service from a battery over its lifetime, the ability to determine the state of charge of a battery is extremely important. State of charge refers to the amount of usable power remaining in a battery at a particular time.
Various techniques have been used heretofore to determine the state of charge of a battery. One common way is to monitor the battery voltage. This method has several limitations. For many types of cells, the open circuit voltage of the battery either does not change with the state of charge (e.g., zinc-mercury oxide battery cell) or changes in steps (e.g., cadmium-silver oxide cell). Clearly, if the voltage is constant while the state of charge varies, voltage cannot be used as an indicator of the state of charge. Furthermore, for all types of battery cells, the voltage tinder load is affected not only by the state of charge but also by the current being drawn from the battery. Therefore, when the battery is in use, monitoring the voltage under load does not give an accurate estimate of the state of charge.
A method of estimating the residual capacity of batteries for electric vehicles is described in Yuasa Jiho 1992, 72, 4-12 (Japan). As described therein, the residual capacity of a battery of an electric vehicle is estimated from open circuit voltage values. The true open circuit voltage value is estimated from a regression line determined by interrupting the discharge current for a very short period of time while the vehicle is operating and carrying out a regression analysis of a subtle change in voltage during the interruption. The true open circuit voltage can also be estimated by carrying out a regression analysis of current and voltage values during operation of the vehicle. Such a method, however, is complex and requires sophisticated instruments.
For nickel hydrogen cells, the state of charge can be estimated accurately from the hydrogen pressure inside the cell. This method, however, is not applicable to any other type of cell.
What is needed, then, is a way of determining the state of charge of a cell whether the cell is in use or not.